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VARIABLE-DISPLACEMENT PUMPS

In servo control systems, the variable-displacement or servo pump performs the same basic function as the control valve. However, the servo pump generates the controlled fluid power as demanded in response to an input control signal. In most applications, the pump is driven at nearly constant speed by an electric motor or some other mechanical power source. The output flow and pressure are then functions of the pump’s displacement or stroke. A radial-piston pump will be used. in explaining the operating characteristics of the variable-displacement pump. The radial-piston pump as shown in Fig. 1.35 has a circular cylinder block which is rotated on a fixed or stationary pintle. The pintle serves both as the bearing for the cylipder block and as the valving or porting member for the inlet and discharging of fluid to the moving cylinder block. The cylindrical pistons bear against the movable race, or reaction ring. When the race is concentric with tile pintle, the pistons have no radial movement, and no fluid is pumped. If the race is moved horizontally to the position, as shown by Fig. 1.35, the pistons move outward during one half-revolution, drawing fluid into the cylinders through the pintle port P 2. During the second half-revolution, the pistons are forced inward, and the . fluid is discharged from the cylinders through the second pintle . port PI. The distance that the pistons move within the cylinders or their displacement, and hence the discharge flow, is proportional to the eccentricity of the race. Movement of the race in the opposite direction from the null or neutral position reverses the direct,ion of the flow in the pintle ports even though the rotation of the pump’s cylinder block is in the same direction.